1. Field of the Invention
This invention relates to a magnetic disk apparatus and a magnetic head control method in which a magnetic head is moved above a rotating magnetic disk.
2. Background Art
The magnetic head positioning control system of a magnetic disk apparatus is typically configured as a digital control system based on a microcomputer. More specifically, discretely obtained position information of the magnetic head is used to calculate a control command in a microprocessor, and the control command is provided to a drive of an actuator through a D/A (digital-to-analog) converter. In general, an actuator has mechanical resonance in a high-frequency band. Hence, to move the magnetic head to a target position with high velocity, low vibration, and low sound noise, it is very important to generate a feedforward control input that does not excite mechanical resonance.
A possible method for rapidly moving a magnetic head for a short distance is to use an optimization technique in previously calculating a feedforward control input to the actuator, which is designed to avoid excitation of mechanical resonance, and a target position command for a feedback control system, and to maintain them as a table. However, such a method is impracticable for covering all the seek distances in view of the memory capacity of the microprocessor. Hence, for a long-distance seek, the feedforward control input to the actuator and the target position command need to be generated online.
In a possible method for this purpose, an actuator model provided in a control system is used to cause a model velocity to follow a target velocity curve so that a control command for the model and a model position are provided to the feedback control system as a feedforward control input to the actuator and a target position command, respectively (e.g., JP-A 9-073618A (Kokai). However, in the head positioning control system disclosed in JP-A 9-073618A (Kokai), the magnetic head unfortunately undergoes overshoot at the settling time unless the position and velocity of the model are close to the actual position and velocity of the magnetic head. Hence some model correction is required during the seek.
In a method for this purpose, during the first half of the seek, the model is updated by applying the feedback control output to the input of the model, the position and velocity of the magnetic head are predicted by the model, and the predicted position and velocity of the model are used to construct a velocity control system (e.g., Proceedings of JSME (Japan Society of Mechanical Engineers) 74th General Meeting, vol. 4, pp. 410-411 (1997) FIG. 3). Furthermore, in this method, during the second half of the seek in which the magnetic head is close to the target position, the feedback control output is switched to the actuator to construct an ordinary two-degree-of-freedom control system.
However, in the case where construction of a servo system is required as in a magnetic disk apparatus, the feedback controller includes an integrator. Hence, when such a seek control system is constructed, the output of the integrator is also applied to the model. Thus, under large external force such as magnetic latching force, the output of the integrator increases. Hence the control command unfortunately produces a transient response upon switching to the ordinary two-degree-of-freedom control system.
Furthermore in the case of a magnetic disk apparatus, when the magnetic head moves rapidly, it obliquely traverses the servo pattern, increasing the position sensing noise. The position sensing noise affects the model-side velocity feedback control system through the feedback control output. Hence, unfortunately, an oscillatory feedforward control command is produced, and also causes sound noise. For these reasons, in a control system such as those described in JP-A 9-073618A (Kokai) and Proceedings of JSME (Japan Society of Mechanical Engineers) 74th General Meeting, vol. 4, pp. 410-411 (1997), it is difficult to realize robust seek control under an environment subjected to large external force. Furthermore, it is also difficult to reduce the effect of position sensing noise.